There are 3 ways in x-ray image observing and recording in the prior art. The first way uses an x-ray film that is still being used nowadays because of its excellent long term reservation property and high resolution. However, there are some disadvantages in the first way. The most obvious disadvantage is that the developing and fixing of the film is too complicate and it is impossible to realize a real time observation. The second way involves a so-called converting screen that consists of (ZnCd)S with activator and is used to convert the x-ray into visible light for observation. The disadvantage of this way is that the efficiency of the converting screen is too low, so that it must be operated in a dark room, and the dose rate of the x-ray must be high enough in order to obtain certain brightness. The third way employs an x-ray image intensifier. The first generation of x-ray image intensifier was developed in the early 1950s which is actually a vacuum photoelectron imaging device built up on the principle of first converting the x-ray image into a visible light image by use of a converting screen, then converting the optical image into electronic image by means of a photocathode and finally focusing the electronic image onto the cathodeluminescence screen by use of an electron optical system to obtain a reduced bright visible image. The image enhancement of the image intensifier is realized by accelerating the photoelectron and reducing the size of the image. In the early stage, the converting screen of the image intensifier was made of zinc surphide and cadmium surphide. From the late 1960s to the early 1970s, a converting screen of CsI(Na) was developed with which both spatial resolution and x-ray converting efficiency are improved significantly. Up to now, this kind of x-ray intensifier is still being used as a key pan of an x-ray TV fluoroscopy system. However, as the electron-optic system of the x-ray image intensifier is complicated and its assembling is very critical and the manufacturing processes of the CsI(Na) converting screen and the whole tube are extremely sophisticated, the imaging quality depends on the stability of power supply too, the whole system itself is a 9 to 12 inches large vacuum device that is very expensive and unportable, this kind of x-ray intensifier is limited to be used in large hospital only.
In 1971-1972, an x-ray intensifier with an x-ray converting screen and a double-proximity focus visible light image intensifier was published in "IEEE Transaction on Electron Devices," Vol. 18, pp. 1101-1108(1971) and "Advances in electronics and electron physics," Vol. 33A, PP. 153-165(1972) by I. C. P. Miller. By means of proximity focus, image distortion of the image intensifier and influence of voltage fluctuation and magnetic field interference on imaging quality are eliminated and the requirement for mechanical assembling accuracy is significantly lowered. Some improvements in manufacturing process of said image intensifier made by Lol Yin was shown in U.S. Pat. No. 4,142,101 in which the converting screen was deposited on the outer side of the input fiber-optical plate of the visible light image intensifier. The common feature of Miller and Lol Yin's x-ray image intensifiers is the use of the combination of an x-ray converting screen with a double-proximity focus visible light image intensifier. A fatal weakness of this x-ray intensifier is that the usable area is limited by the visible light image intensifier and it is difficult to manufacture a visible light image intensifier with large aperture. The apertures of double-proximity focus visible light image intensifier has been still limited 40 mm of diameter nowadays and therefore this patented technology can not be used widely.
A plate x-ray image intensifier was disclosed in U.S. Pat. No. 4,104,516 that is comprised of a converting screen/photocathode and a fluorescent screen with a spacer of 8-20 mm between them and a voltage of 15-60 kv. The advantage of said plate x-ray image intensifier is that its aperture can be made large enough without causing any image distortion. However, since the gain of light is obtained only by accelerating the photoelectron by use of high voltage, gain and resolution of this type intensifier are relatively low. On the basis of U.S. Pat. No. 4,104,516, a type of two stage x-ray image intensifiers in series was built as disclosed in U.S. Pat. No. 4,362,933. The said x-ray image intensifier is formed by inserting an intermediate screen/photocathode component between the input and output screens of the said plate x-ray image intensifier described in U.S. Pat. No. 4,104,516. Because one side of the said intermediate screen is a fluorescent screen and the other side is a visible light photocathode, an x-ray image intensifier consisted of two single stage plate image intensifier in series is formed. As a result, the gain of the intensifier is improved greatly, however, spatial resolution of the unit is further limited by the intermediate screen and the two proximity focus structures.
The common weakness of the reverse image type and plate type x-ray intensifiers mentioned above is the use of combination of an x-ray converting screen and photocathode that involves two additional processes m the course of converting the x-ray into photoelectron. The first one is converting x-ray into visible light while the second is transferring visible light to the photocathode. In image transferring, each process will cause certain noise and other factors that make the resolution worse. In addition, there is an inherent contradiction between the resolution and converting efficiency: when increasing the converting efficiency, it is necessary to increase the thickness of the scintillator, however, with the increasing of the scintillator thickness, dispersion of the visible light occurred in passing through the scintillator increases, making the resolution of the x-ray image intensifier worse. The approach of avoiding the above-mentioned situation is to convert the x-ray directly into photoelectron, i.e. to use an x-ray photocathode instead of the above-mentioned converting screen/photocathode combination.
In "Radiology," Vol. 110, PP. 673-676 (1974), an image intensifier was introduced in which an MCP acts both as x-ray photocathode and electron multiplier. U.S. Pat. No. 3,394,261 introduced a similar technology in which one of the image noise generating links was avoided and the spacer of the proximity focus device that causes image element dispersion was decreased. As a result, the spatial resolution of the image intensifier was increased. However, these inventions were not used widely because the quantum efficiency of MCP in 30-100 kv region of medical x-ray is too low and the MCP could not be made in large size at that time.
In 1976, N. G. Alexandropoulos of rice university in the United States introduced a new type x-ray image intensifier in "Nucl. Instrum. and Methods (Netherlands)" Vol. 137, issue 1, P. 49. The said x-ray intensifier is a test prototype of a proximity focus x-ray image intensifier in which an x-ray photocathode (aluminum, CsI, etc.) is used to convert x-ray directly into photoelectrons and a channel electron multiplier matrix is used to multiply the photoelectrons. In 1979, J. E. Bateman and R. J. Apsimon introduced a CsI x-ray photocathode and a prototype of a double-proximity focus x-ray image intensifier formed by a cathode and MCP in "Advances in Electronics and Electron Physics" Vol. 52, PP. 189-200 (1979). The thickness of the said photocathode equals to the thickness of a 5 .mu.m aluminum foil plus the thickness of 200-500 .mu.m porous CsI layer (.rho.=0.18 gm.sup.-1 cc). The said photocathode can be used to convert x-ray directly into photoelectron and therefore can be used to replace the x-ray converting screen/photocathode combination, as a result, both resolution and sensitivity of the image intensifier are improved. The CsI of such a structure has higher quantum efficiency in dealing with soft x-ray (below 10 kev) than with medical x-ray (energy being 30-100 kev).
In 1987, Tan Kaisheng developed an improved CsI x-ray photocathode as published in "Journal of Electronics of China." The said photocathode is comprised of high/low/high density CsI and has a quantum efficiency as high as 1 to 10 times of that of the high density CsI cathode. In dealing with x-ray below 10 kev, its quantum efficiency is almost the same as the low density one. Moreover the energy of the photoelectrons distributs more concentrically and the spatial resolution is increased. Chinese patent No. 91227072.1 introduced another type of x-ray image intensifier that includes a ceramic envelope, a photocathode comprised of high/low/high density CsI, MCP and a fluorescent screen. By means of this technology, the quantum efficiency of the x-ray photocathode is increased, toughness and spatial resolution of the image intensifier are improved, and the effective area of the image intensifier is enlarged.
U.S. Pat. No. 5,225,670 introduced a similar technology as mentioned above. The x-ray photocathode is of high/low density CsI structure where the density of the low density CsI layer becomes lower and lower from the high density layer to the vacuum. The patent claims that this structure can increase the quantum efficiency of the cathode, however, the rough surface of the porous structure may cause electric discharge and CsI particles come-off.
In the above-mentioned photocathodes which convert x-ray directly into photoelectrons, thick cathode and porous CsI are used for higher quantum efficiency, the photoelectrons generated by the x-ray move random within the CsI layer because of scattering and dispersion, resulting in the deterioration of both spatial and time resolutions.